1. FIELD OF THE INVENTION
The present invention relates to a video camera using a solid-state image sensor, and more particularly to a video camera having an image enhancing circuit.
2. DESCRIPTION OF THE RELATED ART
An image pickup tube has hitherto been widely used as the imaging means of a video camera. The image pickup tube, however, has drawbacks such as an image lag, sticking, and cumbersome adjustment. It has been earnestly desired to overcome such drawbacks. With the advance of semiconductor technology, the performance of a solid-state image sensor such as a charge coupled device (CCD) has been rapidly improved. Thus, the solid-state image sensor has become the mainstream of an imaging device.
A video camera using a solid-state image sensor is usually made up of a driving part, an imaging part, a signal processing part, and an image enhancing part. In the imaging part, a solid-state image sensor converts a light signal indicative of an image into an electric signal, which is delivered from the solid-state image sensor. In a video camera required to form a high-quality image such as a broadcasting camera, the imaging part is made up of a color separation prism and three solid-state image sensors. For example, in the R-G-B method, red, green and blue components are separately taken out of the prism.
In order to take out a signal from each of the solid-state image sensors, the driving part produces a drive signal, and drives the solid-state image sensors. Since a solid-state image sensor used in a video camera is an area sensor, the solid-state image sensor is usually driven in two directions, that is, horizontal and vertical directions.
The signal processing part carries out various signal processing which includes signal amplification and .gamma.-processing. The image enhancing part amplifies a high-frequency region of a video signal, and thus can be regarded as a portion of the signal processing part. Owing to the above operation, the contour portion of an image is made clear, and a distinct image is obtained. Image enhancement is usually carried out in horizontal and vertical directions. In order to carry out the image enhancement in the vertical direction, not only the signal from a target pixel but also the signals from pixels adjacent to the target pixel on the upper and lower sides are required. Further, a delay circuit for delaying a signal by one horizontal period is required.
In order to simplify the image enhancing circuit and to carry out the effective enhancement, the out-of-green method is used. According to this method, a contour signal is produced only from the video signal of a G-channel, and is then added to the video signal of each of R-, G- and B-channels to carry out image enhancement. The video signal of the G-channel used for forming the contour signal is delayed by one horizontal period. Accordingly, it is necessary to delay the video signals of the R- and B-channels by one horizontal period.
In other words, even when the out-of-green method is used, a conventional video camera is required to have two 1-horizontal-period delay circuits for the G-channel and to have one 1-horizontal-period delay circuit for each of the R- and B-channels. That is, the video camera is required to have four 1-horizontal-period delay circuits. In an analog system, a glass delay line is used as the 1-horizontal-period delay circuit. In a digital system, a semi-conductor memory is used as the 1-horizontal-period delay circuit. In either case, the video camera will become large in power consumption and circuit scale. That is, the fact that there are not a few 1-horizontal-period delay circuits in the image enhancing circuit, will be a serious obstacle to the production of a camera head with a built-in, image enhancing circuit.
The number of 1-horizontal-period delay circuits included in the image enhancing part can be reduced by shifting the relative optical position of the solid-state image sensors in the vertical direction. In this case, on the output side of the signal processing part, the R- and B-channels are always delayed as compared with the G-channel, by one horizontal period. That is, it is unnecessary to use a 1-horizontal-period delay circuit for each of the R- and B-channels. This method, however, is not applicable to image enhancing circuits other than the image enhancing circuit for exclusive use of the above video camera. For example, in a case where desired coefficient matrix processing is carried out in the R-, G- and B-channels, and the-signal thus obtained is used for image enhancement, it is required that there is not any time difference among the R-, G- and B-channels. Therefore, the method of shifting the relative optical position of solid-state image sensors is not applicable to an image enhancing circuit used in the above case.